In recent years, in transmission of video data and audio data on the Internet, there are down-load type transmission system and stream type transmission system. In this down-load type transmission system, video file which has been transmitted from distribution server is temporarily stored at the terminal side, and data (video data) of video file is then reproduced. For this reason, in this system, until transfer of file is completed, it is impossible to reproduce data at the terminal side. The down-load type transmission system is unsuitable for use in long hour reproduction of video data, etc.
On the other hand, in the stream type transmission system, also for a time period during which data is transmitted from a streaming server to the terminal, received data is reproduced. In this stream type transmission system, system using protocol called RTP (Real-Time Transport Protocol) prescribed in the RFC 1889 of IETF (Internet Engineering Task Force) is the main current.
Meanwhile, in recent years, there has been great demand for security countermeasure by encryption communication. For example, there has been proposed IPSec (IP Security Protocol) for encrypting, at IP (Internet Protocol) level, all communication data transmitted from Host without merely encrypting only communication data by a specific application, etc. This IPSec is standard requirements for encryption communication that the IETF is proceeding with standardization.
In this IPSec, since padding or padding size, etc. caused to be in correspondence with block size is added to payload data to thereby perform encryption communication, there is the problem that encryption/decoding is required at the communication portion of the transmitting side and the receiving side so that mounting by the existing distribution tool is difficult.
In the case where padding or padding size is added, size of encrypted data becomes greater than actual data, so change of size takes place before and after encryption. For this reason, there was the problem that bit rate of stream and/or file size change. In addition, since the number of data transmitted by streaming becomes large, there is the problem that performance of streaming is lowered.
It is also conceivable to encrypt image data which has been compression-encoded by the MPEG4 (Moving Picture Experts Group 4) to perform streaming of the encrypted image data. Here, in the MPEG4, it is prescribed at RFC3016 that data is transmitted in video packet units at the time of streaming. However, since break of video packet and break of encryption block do not coincide with each other, encryption cannot be completed in video packet units. In the case where video packet is missing by transmission error, etc., there is the problem that not only that video packet, but also video packets before and after cannot be decoded.